1. Field of the Invention
This invention relates generally to communication systems in an electric power system, and more specifically to a relay-to-relay direct communication system and method in an electric power system.
2. Description of Related Art
In U.S. Pat. No. 5,793,750, the contents of which are hereby incorporated by reference, a communication system between two microprocessor-based protective relays for an electric power system is disclosed. Each of the two relays in that system has both transmit and receive modules, for directly transmitting indication status bits indicative of the result of selected protective functions of one relay from that one relay to the other, and vice versa.
The output status indication bits are sometimes used to identify the existence and location of a fault on the power line portion served by the two relays. One or both of the relays might initiate a circuit breaker trip action on the basis of the exchange of such information. The output status indication bits may be the result of processing functions in one of the relays involving the voltages and/or currents on the power line. The output status indication bits may be used for various control, status, indication and protection functions. Examples of protection functions include permissive overreaching transfer trip (POTT) actions, permissive under-reaching transfer trip (PUTT) actions, directional comparison unblocking (DCUB) and direct transfer trip (DTT) actions. Other relay-to-relay operations are possible using particular output status indication bits.
The advantage of the communication system described in the '750 application is that it is fast and secure. Protective relays typically accomplish their monitoring functions several times each power system cycle. The '750 communication system provides the results of these monitoring functions of one relay, to the other relay. The information is transmitted directly over a communications link from an originating relay which may or may not trip its associated circuit breaker based on its operational results, to another relay. The receiving relay then uses the transmitted information, in the form of digital bits, to perform its own on-going calculations, producing various protection actions such as tripping and closing a circuit breaker when appropriate. The communication between the two relays may be bidirectional, allowing the two relays to exchange information concerning the results of their own calculations both quickly and securely, with a minimum amount of expense.
In the '750 application, the output status indication capability is eight bits. In many cases, however, eight channels are not necessary. For example, two or three bits are usually sufficient to accomplish the desired relay-to-relay protection, control, and monitoring scheme. A substantial number of bits therefore may go unused. The present invention makes use of those otherwise unused bits. It forms a serial data stream or channel from each unused bit, and utilizes those serial data channels to significantly increase the amount of information that can be communicated between the two relays. This invention may utilize none, some or all of the eight channels to transfer output status indication bits. If less than all eight channels are used for output status indication bits, the otherwise unused channels may be used to transfer other selected information.